Short answer: A tripeptide is a peptide built from three amino-acid residues. In a linear tripeptide those three residues are held together by two peptide (amide) bonds, and each bond forms with the elements of water removed [1]. The name and the sequence are written from the N-terminal residue to the C-terminal residue, so the same three amino acids can give several different tripeptides [1][2]. Well-known examples include glutathione and triglycine (Gly-Gly-Gly) [1][3][4].
This article is educational chemistry for laboratory readers. It does not describe biological effects, and it is not guidance for use in people or animals.
Tripeptide definition
IUPAC-IUB's Joint Commission on Biochemical Nomenclature defines a peptide as any compound made by amide formation between a carboxyl group of one amino acid and an amino group of another, and says the amide bonds in peptides may be called peptide bonds [1]. A multiplicative prefix placed before "peptide" gives the total number of residues [1]. "Tri-" therefore means three residues.
When amino acids join, what is left of each one is called an amino-acid residue [1]. A tripeptide has three residues:
- an N-terminal residue, whose amino group is free (or at least not acylated by another residue) [1];
- one internal residue in the middle;
- a C-terminal residue, whose carboxyl group is free (or at least does not acylate another residue) [1].
How many peptide bonds does a tripeptide have?
Two, in the usual linear form. Each new residue adds one amide bond, and OpenStax describes the chain this way: the amino group of one residue bonds to the carboxyl of a second, the amino group of the second bonds to the carboxyl of a third, and so on [2].
Because water is removed at every bond [1], a tripeptide's molecular weight is the sum of its three amino acids minus two waters. Triglycine is an easy check:
- Glycine: 75.07 g/mol [5]
- Water: 18.015 g/mol [6]
- 3 × 75.07 − 2 × 18.015 = 189.18 g/mol
- PubChem lists glycylglycylglycine at 189.17 g/mol [4]; the small difference is rounding.
How tripeptides are named
IUPAC builds peptide names from acyl names ending in "-yl" [1]:
- The name starts with the acyl name of the N-terminal residue.
- Internal residues follow in order, also as acyl names.
- Only the C-terminal residue keeps its full amino-acid name, and it ends the name [1].
Formulas are written in the same order, with the N-terminal residue on the left and the C-terminal residue on the right [1][2]. Configurational prefixes such as L- and D- go directly before the residue they describe, for example L-alanylglycyl-L-leucine [1]. If you need a refresher on the symbols themselves, see our guide to amino acid codes.
Order matters: one set of amino acids, several tripeptides
Swapping two residues gives a different compound. OpenStax shows this for two amino acids: alanylserine (Ala-Ser) and serylalanine (Ser-Ala) are different dipeptides [2]. With three different amino acids the number grows: OpenStax notes that six isomeric tripeptides contain valine, tyrosine and glycine [2]. That is simply 3 × 2 × 1 = 6 possible orders:
- Val-Tyr-Gly (VYG)
- Val-Gly-Tyr (VGY)
- Tyr-Val-Gly (YVG)
- Tyr-Gly-Val (YGV)
- Gly-Val-Tyr (GVY)
- Gly-Tyr-Val (GYV)
Tripeptide examples
Glutathione. IUPAC gives glutathione as γ-glutamylcysteinylglycine [1]. The "γ" matters: the glutamic acid residue is joined through its side-chain (γ) carboxyl rather than its usual α-carboxyl [1][7]. IUPAC calls amide links between residues other than the usual C-1 to N-2 connection isopeptide bonds, and it still counts such compounds as peptides [1]. PubChem lists glutathione as C10H17N3O6S, 307.33 g/mol [3].
Triglycine (Gly-Gly-Gly). The simplest tripeptide: three glycine residues. PubChem lists it as C6H11N3O4, 189.17 g/mol [4][12].
Thyroliberin (Glp-His-Pro-NH2). IUPAC uses this tripeptide as a notation example [7]. The -NH2 at the end of the symbol marks an amide at the C-terminal residue [11]. PubChem lists the compound as protirelin; its systematic name contains a 5-oxopyrrolidine-2-carboxamide group (a ring) at one end and a carbamoylpyrrolidine group at the other, and its formula is C16H22N6O4, 362.38 g/mol [8].
GHK (glycyl-L-histidyl-L-lysine). A tripeptide with glycine at the N-terminus and lysine at the C-terminus. PubChem lists it as C14H24N6O4, 340.38 g/mol [9].
Tripeptides, oligopeptides and polypeptides
Where does a tripeptide sit among the larger terms?
- IUPAC says peptides with fewer than about 10–20 residues may also be called oligopeptides, and those with more are polypeptides [1].
- IUPAC adds that polypeptides of specific sequence with more than about 50 residues are usually called proteins, though authors differ on where that term starts [1].
- NHGRI describes a peptide as a short chain of typically 2 to 50 amino acids, and a chain of 51 or more as a polypeptide [10].
Why this matters when reading a label or COA
- Sequence direction is fixed. Read it N-terminus to C-terminus; Gly-His-Lys and Lys-His-Gly are different compounds [1][2].
- Mass reflects two lost waters. A listed molecular weight should match the residue sum, not the sum of three free amino acids [1][4][5][6].
- Prefixes and symbols carry structure. L-, D-, γ-, Glp and -NH2 each tell you something specific about configuration, linkage or end groups [1][7][11].
Frequently Asked Questions
What is a tripeptide in simple terms?
Three amino-acid residues linked by peptide (amide) bonds, with water removed as each bond forms [1].
How many peptide bonds are in a tripeptide?
Two in a linear tripeptide: one between residues 1 and 2, and one between residues 2 and 3 [2].
How many different tripeptides can three different amino acids make?
Six, one for each order. OpenStax's example uses valine, tyrosine and glycine [2].
Is glutathione a tripeptide?
Yes. IUPAC names it γ-glutamylcysteinylglycine: three residues, with the glutamyl residue linked through its γ-carboxyl [1].
How do you calculate a tripeptide's molecular weight?
Add the three amino-acid molecular weights and subtract two waters. For triglycine: 3 × 75.07 − 2 × 18.015 ≈ 189.18 g/mol, in line with PubChem's 189.17 [4][5][6].
What is the difference between a dipeptide and a tripeptide?
A dipeptide has two residues and one peptide bond; a tripeptide has three residues and two. The prefix gives the residue count [1].
Bottom line
A tripeptide is three amino-acid residues joined, in the linear case, by two peptide bonds formed with loss of water [1][2]. Its name and sequence run from the N-terminus to the C-terminus, so residue order defines the compound [1][2]. Glutathione, triglycine, thyroliberin and GHK show how symbols, linkages and end groups are recorded [1][3][4][7][8][9][11][12].
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